CMOS battery; trying to set UNSTBL_PWR
I did a bit of fiddling last night with the CMOS battery. Wasn't sure where to put stuff (didn't find much), so I figured I'd do a writeup here and someone else can decide what to integrate into the main page/investigate further/delete/whatever.
Taking the name on face value, I rigged up some digital potentiometers and a simple program to pulse them on and off. Disconnected from the console, it went from 3V to something like 0.1V at a maximum of 4Hz (although I could probably optimise it further). Suffice to say, I tried a lot of stuff with that setup and nothing worked. I may have seen a flicker on my monitoring program to imply that the bit was set, but I was unable to replicate it. I was also pretty tired at the time so it may have been an illusion. Hey, at least I set LOW_BATT reliably ;D
Having given up there, I took a quick look at the battery port without my circuit attached, and was surprised to find some power there. The exact voltage drifts (usually between 7 and 10V, although I saw it get up to 25 at one point), but it's always AC at 50Hz (bang-on 50Hz, too).
I think this is why I didn't get much success - 3V DC variation isn't going to make much impact on that. A possible thing to try is unplugging the console, forcing the RTC to use the battery, although you obviously can't monitor whether you've had any success. I'm honestly not sure it's worth it at this point - better off just using a hardmod imho.
Side note: Shorting the +/- pins of the CMOS battery sets LOW_BATT without any extra circuitry.